Facile in-situ synthesis of NbB 2 nanoparticles at low temperature Aayush Gupta, Varun Singhal, O.P. Pandey * Functional Materials Lab, School of Physics and Materials Sciences, Thapar University, Patiala, 147004, India article info Article history: Received 28 July 2017 Received in revised form 1 October 2017 Accepted 27 October 2017 Available online 28 October 2017 Keywords: Niobium diboride Nanoparticle Williamson-Hall analysis Synthesis mechanism abstract Niobium diboride (NbB 2 ) nanoparticles have been successfully synthesized at 800  C in a single step from niobium penta-oxide (Nb 2 O 5 ), magnesium (Mg) and borax (Na 2 B 4 O 7 $10H 2 O) in an autoclave. The syn- thesis temperature and holding time have been optimized to obtain single phase NbB 2 . The phase determination, thermal stability and morphological features of synthesized samples have been analyzed by X-ray diffraction (XRD), thermal gravimetric-differential scanning calorimetry (TG-DSC) and electron microscopic technique respectively. Williamson-Hall (W-H) analysis has also been done to observe the effects of synthesis parameters (temperature and holding time) on crystal distortion. With the help of XRD, microstructural features and thermodynamic calculations, formation mechanism for the synthe- sized samples has been predicted. © 2017 Elsevier B.V. All rights reserved. 1. Introduction Because of multiple high end applications of transition metals, their compounds (oxides, carbides, nitrides and borides) and composites, new processes are being developed to synthe- size them at low temperatures. High melting point, chemical stability alongwith the high strength and wear resistance make these materials suitable for various engineering applications [1e4]. Moreover, their electronic structure exhibits superior superconducting and catalytic properties [5,6]. Among all these compounds, transition metal borides (TMBs) are considered as very good candidates for refractory, superconducting and me- chanical applications due to their versatile performance [7e12]. Moreover, among all TMBs, niobium diboride (NbB 2 ) holds a significant place due to its distinct physical and chemical prop- erties as it possesses high melting point (~2900  C) with low density (~6.97 g/cm 3 ) [9]. It is also used as electrodes for refining of aluminum [13], in hypersonic aerospace vehicles [9], in high speed cutting tools [9] and as superconducting material [12]. For all the engineering applications as mentioned above, density of sintered ceramic compound is a very critical parameter which is a function of initial particle size of the powder. It becomes pre- requisite to obtain small particle size (nano scale) of NbB 2 which exerts higher capillary force during sintering, so that porosity of the sintered dense ceramics can be minimized [14]. NbB 2 has been synthesized by various methods including bor- othermal reduction, high pressure or high-temperature technique, chemical vapor deposition, self-propagating high-temperature synthesis, physical vapor deposition, inorganic solvent thermal route and mechano-chemical process [12,15e20]. All the above mentioned routes are either high temperature, high pressure, multi-step methods or require high-end equipment to obtain NbB 2 at nanoscale. In most of the studies, NbB 2 has been synthesized at very high temperature [2,15,19,21] while, Ma et al. [18] have ach- ieved the same at low temperature (650  C). However, the synthesis process involves complex reaction route in glovebox in which inorganic solvent (NaCl þ MgCl 2 ) was used to provide better control over the diffusion controlled reaction rate under inert atmosphere. Although the synthesis temperature is quite low, but the optimi- zation of synthesis parameters, microstructural features and detailed thermal behavior of synthesized NbB 2 were not discussed in the reported work [18]. Moreover, diffusion of boron for the formation of NbB 2 is dependent on the reduction kinetics of Nb 2 O 5 . In our present study, solid state reaction has been carried out inside the autoclave with the help of inorganic mixture of Nb 2 O 5 (Nb source), Mg (reducing agent) and borax (B source) at different temperatures for different durations and also by varying the amount of reducing agent to understand the detailed reaction mechanism responsible for transformation of Nb 2 O 5 to NbB 2 nanoparticles at such a low temperature. * Corresponding author. E-mail address: oppandey@thapar.edu (O.P. Pandey). Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom https://doi.org/10.1016/j.jallcom.2017.10.257 0925-8388/© 2017 Elsevier B.V. All rights reserved. Journal of Alloys and Compounds 736 (2018) 306e313